This invention relates to a method of and apparatus for continuously producing semiconductor devices on a substrate by depositing successive layers of semiconductor alloy material in a plurality of operatively interconnected glow discharge deposition chambers. The composition of each layer of semiconductor alloy material is dependent upon, inter alia, (1) the particular precursor mixture of process gases introduced into each of the deposition chambers, (2) the method of forming the semiconductor alloy materials from those process gases, (3) the length of time those process gases are subjected to the electromagnetic field utilized to decompose said gases, and (4) the strength and uniformity of that electromagnetic field. More particularly, (1) the precursor mixture of process gases introduced into the first deposition chamber is carefully controlled and isolated from the precursor mixture of process gases introduced into adjacent deposition chambers; (2) the electromagnetic field is maintained at a constant, uniform strength over the entire length of the cathode; (3) the mixture of process gases flowing through the plasma region is prevented from being depleted along its path of travel; and (4) areas of stagnation are removed from the path of travel of those process gases through the plasma region of the deposition chamber. In this manner, homogeneous layers of semiconductor alloy material may be successively deposited onto a large area substrate in each one of the deposition chambers. If each individual one of the layers of semiconductor alloy material is not uniformly and homogeneously deposited, the overall efficiency of the semiconductor device produced as a conglomeration of those layers suffers. It therefore becomes necessary to carefully control all processing steps which bear on the uniformity, homogeneity and general quality of the deposited semiconductor alloy material.
In the glow discharge deposition of semiconductor alloy material onto a substrate, the discrete mixtures of process gases introduced into each of the dedicated deposition chambers is directed to flow between a large area cathode plate and the electrically conductive, large area substrate spaced thereabove. It is in this area, bounded by the cathode and substrate, and referred to hereinafter as the plasma region, that a.c. or d.c. energy coupled to the cathode, develops an electromagnetic field which operates to disassociate the mixture of precursor process gases into activated species which are then deposited onto the substrate. If the electromagnetic field is not uniform over the entire length and width of the cathode, the properties of the semiconductor alloy material depositing onto the substrate will be accordingly affected. More particularly, nonuniform power densities of electromagnetic energy created in discrete portions of the plasma region result in the deposition of layers of nonuniform and nonhomogeneous semiconductor alloy material. Therefore, the deposition of nonuniform, nonhomogeneous, semiconductor alloy material, which nonuniformity and nonhomogeniety is caused by the presence of gradients in the strength of the discrete electromagnetic field established within the single plasma region, must be substantially prevented.
It has been discovered that the uniformity and homogeneity of a layer of semiconductor alloy material deposited onto the substrate at the portions of the plasma region proximate the opposite distal ends of the large area cathode varies from the uniformity and homogeneity of the layer of semiconductor alloy material deposited onto the substrate inward of those end portions (i.e., the central portion of the plasma region). This discovery is disclosed in U.S. patent application Ser. No. 418,859, filed Sept. 16, 1982, which application is assigned to the Assignee of the instant application and the disclosure of which is incorporated herein by reference.
By way of illustration, and referring to the schematic drawing of FIG. 3, Arrow 9 indicates the direction of movement of the grounded web of substrate material 11, which web is spaced above the cathode plate 34 a distance "d" so as to define a plasma region 80 therebetween. It is in this plasma region 80 that a mixture of process gases are disassociated and recombined into activated species. In the exemplary plasma region 80 depicted in FIG. 3, two discrete electromagnetic fields which vary in intensity of electromagnetic energy are illustrated.
The nature of the electromagnetic field generated between the cathode 34 and the substrate 11 determines the reaction kinetics for the glow discharge plasma created therebetween. Accordingly, the properties of the layer of semiconductor alloy material deposited from the mixture of process gases decomposed, recombined and activated in the centrally disposed electromagnetic field labelled "A" are different from the properties of the layer of semiconductor alloy material deposited from the mixture of process gases decomposed, recombined and activated in the distally disposed electromagnetic fields labelled "B". It should therefore be readily apparent that the presence of such nonuniform electromagnetic fields at the distal ends of the plasma region 80 causes serious problems with respect to the deposition of uniform and homogeneous layers of semiconductor alloy material.
Still referring to FIG. 3, note that the electromagnetic field A, disposed centrally within the plasma region, is substantially uniform and homogeneous. However, in comparison thereto, the electromagnetic fields B, disposed at the opposed distal portions of the plasma region are substantially nonuniform and nonhomogeneous. This difference in uniformity and homogeneity of the electromagnetic field is due to the fact that the electromagnetic field A is developed in the central portion of the plasma region, a portion of the plasma region in which the distance between the substrate and the cathode is constant, therefore forming electromagnetic field vectors which are substantially perpendicular to the plane of the substrate. In contrast thereto, electromagnetic fields B are developed at the distal portions of the plasma region, the portions of the plasma region which are adjacent the opposed distal ends of the cathode plate 34 and in which the substrate-cathode distance varies, thereby promoting angled or bent electromagnetic field vectors relative to the plane of the substrate. The result of the nonuniform and nonhomogeneous electromagnetic fields B is the deposition of a layer of semiconductor alloy material which is nonhomogeneous and nonuniform in cross-section onto the surface of a substrate traveling through the plasma region.
FIG. 4 illustrates shielding apparatus disclosed in said previously filed U.S. patent application Ser. No. 418,859, said shielding apparatus adapted to prevent nonhomogeneous, nonuniform semiconductor alloy material produced by the presence of the distally opposed, nonuniform and nonhomogeneous electromagnetic fields B from being deposited onto the surface of the web of substrate material 11. The shielding apparatus includes a pair of substantially identical shielding plates 60, one of which is positioned adjacent each of the opposed distal ends of the cathode plate 34. It should be apparent that, although only a single cathode is shown in the schematic drawing of the depositon chambers of FIG. 2, a plurality of discrete cathode plates may actually be employed in an elongated, single depostion chamber. It is to be understood that in such instances, a shielding plate 60 is to be positioned adjacent each of the opposed distal end of each discrete cathode plate 34.
FIG. 4 illustrates the manner in which the shielding plates 60 function to blanket the web of substrate material 11 in the areas of nonuniform, nonhomogeneous electromagnetic fields B so that the ionized plasma developed from the mixture of process gases flowing through the plasma region is deposited onto the shielding plates 60 rather than onto the web of substrate material 11 which travels therepast in the direction of travel of Arrow 9. In order to best accomplish that web-blanketing function, the shielding plates 60 are positioned in a plane generally parallel to the plane of travel of the web of substrate material 11 but removed downwardly therefrom by a distance of approximately one-quarter (1/4) inch (approximately the dark space).
A polyimide film 61 may be applied so as to cover at least that surface of each of the shielding plates 60 which faces the web of substrate material 11. The polyimide film is adapted to prevent a glow discharge plasma from developing in the area between the web of substrate material 11 and the shielding plates 60. The polyimide film may be KAPTON (registered trademark of Dupont Corporation), type 4. Polyimide films were selected because of their inherent properties of high temperature stability, good wear resistance at high temperature and low outgassing in high vacuum. Other films could also be used, if the other films exhibit similar properties.
The shielding plates 60 proved to be substantially effective in preventing the deposition of nonuniform and nonhomogeneous semiconductor alloy material under normal operating conditions of Applicants' glow discharge deposition apparatus. However, Applicants have recently discovered a method of depositing a fluorinated microcrystalline p-doped semiconducor alloy material (said microcrystalline p-doped semiconductor material disclosed in U.S. patent application Ser. No. 667,659 filed Nov. 2, 1984, which application is also assigned to the assignee of the instant application and the disclosure of which is incorporated herein by reference), which method requires, inter alia, a significant increase in the level of r.f. power delivered to the cathode plate 34, and hence, a corresponding increase in the strength of the electromagnetic field and the energy imported to the plasma generated in the plasma region.
Since the terms "amorphous" and "microcrystalline" will appear throughout this specification, it is necessary to provide specfific definitions for these terms. The term "amorphous", as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. As used herein the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occurs. It is to be noted that pursuant to the foregoing definitions, the microcrystalline, p-doped semiconductor alloy material referred to in the instant invention falls within the generic term "amorphous".
Referring now to FIG. 5, the centrally disposed activated plasma of a glow discharge deposition chamber specifically adapted for depositing said fluorinated microcrystalline p-doped semiconductor alloy material is illustrated by the reference character A. Since, in order to deposit the microcrystalline p-doped material the electromagnetic field A must be stronger than the electromagnetic fields utilized to deposit amorphous p-doped semiconductor alloy material, a more energetic plasma is generated from the mixture of process gases introduced into the plasma region of the glow discharge deposition chamber. It has been experimentally determined that this highly energetic plasma tends to creep into the space formed between the horizontally disposed plate 35a of the cathode shields 35, which shields are disposed at each of the opposed distal ends of the plasma region adjacent the web of substrate material 11. The creep of the highly energetic plasma into that space is depicted by the reference characters A'. Not only is the "creeping" plasma A' nonuniform and nonhomogeneous, as compared to the plasma A in the centrally disposed plasma region, but that highly energetic plasma A' has been determined to actually cause the semiconductor alloy material deposited in the centrally disposed plasma region to be etched away at a rate far greater than the rate at which semiconductor alloy material is deposited in that centrally disposed plasma region (the semiconductor alloy material is etched at a rate of at least 450 .ANG. per minute).
In an effort to prevent the highly energetic plasma from entering the region between the horizontal plates 35a of the cathode shields 35 and the substrate 11, Applicants coated the surface of those horizontal plates 35a which faces the substrate 11 with the polyimide insulating coating 61 previously described. However, it was found that an electric current from electrons in the plasma was developed at the inner (closest to the central plasma region) edge of the polyimide coating, which current was responsible for drawing the plasma into the region between the plates 35a and the substrate 11. As the plasma was drawn into that region, a layer of silicon was deposited upon the Kapton coating 61, which silicon layer caused the coating 61 to lose its insulating function.
It is therefore a principle object of the present invention to (1) confine the highly energetic plasma, whether developed during the deposition of microcrystalline, p-doped semiconductor alloy material or during the deposition of amorphous material, to the central portion of the plasma region and (2) prevent that highly energetic plasma from creeping between the horizontal plates 35a (as well as any insulating coating disposed thereupon) of the cathode shields 35 and the web of substrate material 11.